Transmitting antenna for audio induction communication system



United States Patent Inventor App], N 0. Filed Patented AssigneeTRANSMITTING ANTENNA FOR AUDIO INDUCTION COMMUNICATION SYSTEM PrimaryExaminer Kathleen H, Claffy Assistant Examiner-W. A. I-lelvestineAtzorneyMc Cormick, Paulding and Huber lsclalms6nrawing Flgs' ABSTRACT:A transmitting antenna in an audio induction [1.8. CI. 179/82,communication system consists of a core of silicon grain 343/787,343/873 oriented steel made of laminations bonded to one another, a Int.Cl. H01g 7/06, shrink fitted plastic insulating sleeve, and a coil ofmagnet wire I-I01g 1/40, H04g 5/00 wound on the sleeve, all of which arereceived in a nonmag- Field of Search 179/82; netic plastic shell filledwith cured silicon rubber as a potting 343/787, 788, 873 agent.

TAPE TRANSPORT AMPLIFIER TRANSFORMER AMPLIFIER EARPHONE PATENTEnnzcazlamm y 3,549,818

v EARP-HONE I NTOR. JUSTIN TURNER ATTORNEYS TRZQSEOR 1 TRANSFORMERAMPLIFIER Y TRANSMITTING ANTENNA FOR AUDIO INDUCTION COMMUNICATIONSYSTEM CROSS REFERENCE TO RELATED APPLICATION BACKGROUND OF INVENTIONThis invention relates to wireless communication systems of the typewherein audio frequency messages are transmitted through space by amagnetic field the intensity of which is varied at such audiofrequency,and deals more particularly with an improved transmittingantenna and associated components for establishing the magnetic field;

In the past, various different systems have been proposed for providingwireless communication by a magnetic field the strength of which variesin accordance with an audio signal so that a coil located in such fieldhas induced therein a signal which may be converted to audible sound bymerely amplifying it and applying it to an earphone, loudspeaker orother similar electromechanical transducer. Such systems are limited tocommunication over relatively short distances, because of the nature ofthe magnetic field, and are, therefore, generally proposed for usewithin relatively small areas such as the rooms of a museum where thesystem may be used to supply messages concerning various exhibits ordisplays to visitors carrying suitable receivers. One major drawback ofprior audio induction communication systems, however, particularly withregard to usage in museums and the like, has been the problem ofcrosstalk between two or more simultaneously broadcast messages due tothe magnetic field of one message interferring with the magnetic fieldof another message. In an audio induction communication systemtransmitting two or more messages simultaneously the field as sociatedwith each message must occupy its own exclusive space without overlap orinterference from the field of another message. Otherwise, a receiverlocated in the zone of overlap-will tend to pick up both-messages.Transmitting antennas in the past have generallybeen in the form ofrelatively large loops of wire, often extending around the periphery ofa room, and to avoid overlap and interference it has usually beennecessary to locate the field of one program at a great distance fromthe field of another program. In museums, for example, it has generallybeen necessary to allot only one magnetic field or message to each room.

The general object of this invention, therefore, is to provide animproved means for establishing the varying magnetic field of an audioinduction communication system and whereby such field maybe controlledso as to allow a number of fields or messages to be placed close to oneanother without undue overlap or interference from one another, therebypermitting a number of fields or messages to be used at once in a singleroom or other small area. I

BRIEF DESCRIPTION OF THE INVENTION The invention resides primarilyin theconstruction of a transmitting antenna used in combination with an audiosignal source and amplifier of an audio induction communication system.The antenna is a coil of magnet wire wound on a core of magneticmaterialwhich is preferably made up of silicon grain oriented steellaminations bonded to one another and surrounded by a shrink fittedplastic insulating sleeve. This unit is potted in liquid silicon rubberwhich is cast into a nonmagnetic container or shell and held undervacuum for a number of hours during curing to eliminate all voidsbetween the convolutions and layers of the wire and thecore to preventchattering and hum as a result of wire vibration. The bonding of thecore laminations to one another also eliminates noise arising fromvibration of the laminations relative to one another.

The length and cross-sectional shape of the core, and the power supplied'to the coil, control the shape of the magnetic field projected from theantenna. The length of the core in general determines the length of thefield axially of the core and the cross-sectional shape determines thewidth of the field radially of the core. The power supplied to theantenna controls both the width and length of the field. Accordingly, by

properly selecting the length and cross-sectional shape of the antennaand the power supplied thereto, the shape of the space occupied by thefield projected. therefrom may be relatively accurately controlled so asto place the field in a given desired location, thereby making itpossible to arrange a number of different fields in a small area so asto not interfere with one another.

BRIEF DESCRIPTION OF-TI-IE DRAWINGS FIG. 1 is a block diagramillustrating an audio induction communication system of the type withwhich this invention is concerned.

FIG. 2 is alongitudinal sectional view taken through the transmittingantenna of FIG. 1.

FIG. 3 is a transverse sectional view taken on the line 3-3 of FIG. 2.

FIG. 4 is a view showing a portion of FIG. 3 in an enlarged scale andwith the spacing between the core laminations exaggerated to reveal thebonding agent used in the core.

FIG. 5 is a diagram illustrating the nature of the field produced by anantenna such as that of FIG. 2 having a relatively large length incomparison to its cross-sectional area.

FIG. 6 is a diagram generally. illustrating the nature of thefield'produced by an antenna such as that of FIG. 2 having a relativelyshort length in comparison to its cross-sectional area.

DETAILED DESCRIPTION OFTHE PREFERRED EMBODIMENT Turning to the drawing,and first referring to FIG. 1, this FIG. shows in block diagram form anaudio induction communication system utilizing a transmitting antenna ofthe type with which this invention is concerned. The illustrated systemincludes an audio signal source in'the form of a magnetic tape transportor recorder 10. The audio signal output of the tape transport istransmitted to an associated power amplifier 12 which produces anamplified audio frequency signal on its output line. The output signalof theampljfier 12 is in turn transmitted to a matching transformer 14having a transmitting antenna 16 connected to the output terminalsthereof. In a typical museum installation, for example, the tapetransport 10 and amplifier 12 may be located at a distribution centerand be only one of many such transport and amplifier pairs used toprovide a large number of messages transmitted simultaneously todifferent areas of the museum. The matching transformer 14 is preferablylocated close, to the exhibit or display with which the transmittingantenna I6 is associated, and it preferably includes a number ofdifferent taps of different impedance, such as 2, 4, 6 and 8 ohm taps,to allow the antenna 16 to be any one of a number of corresponding inputimpedances. The antenna 16, as explained in more detail hereinafter,produces a magnetic field which varies in strength in accordance withits audio input signal and which passes from one end of the antenna tothe other as indicated by the flux lines 18. g

The tap transport 10, or other audio'signal source, amplifier 12,transformer 14 and antenna 16 constitute the transmitting portion of thecommunication system. The receiving portion of the system comprises areceiving antenna 20, an audio amplifier 22 and an earphone 24, or othersimilar electromechanical transducer, for converting the electricalsignals from the amplifier 22 into sound. The complete receiving unitmay bein the form of a relatively small package worn or carried by aperson visiting the museum or other area within which the communicationsystem is located and as the person moves to a point within the fieldproduced by the transmitting antenna 16 the field induces an audiosignal in the receiving antenna which is amplified by the audioamplifier 22 and converted to sound by the earphone 24. The receivingantenna 20 may take various different forms and in general constitutes acoil or loop of wire.

FIGS. 2 and 3 show in detail the construction of the transmittingantenna 16 of the FIG. 1 system. As mentioned, previous audio inductioncommunication systems have generally utilized transmitting antennas inthe form of relatively large loops of wire creating relatively largemagnetic fields, in terms of the space occupied thereby, and most oftenthese antennas have surrounded the perimeter of the room or other areawithin which the transmitted message is to be heard. In contrast tothis, the antenna 16 of this invention is designed to produce arelatively small magnetic field of a readily controlled shape and size.This antenna comprises a core 26 of magnetic material surrounded by acoil 28 of wire having two leads 30 and 32 for connection to thetransformer 14 or other output device. The core 26 is in the nature of arelatively straight bar of substantially uniform cross sectionthroughout its length. It may be made in various different ways and ofvarious different materials but preferably is rectangular in crosssection and consists of a plurality of laminations of silicon grainoriented steel with each lamination extending the full length of thecore and having its grain oriented so as to be generally parallel to thelongitudinal axis of the core. Due to the varying nature of the fieldproduced by the antenna, it tends to set up varying magnetic forcesbetween the laminations of the core, and unless the laminations are heldtightly to one another, they will tend to vibrate and produce anobjectionable audible noise in the formof chatter, hum or an actual lowvolume reproduction of the transmitted message. In some instances, asheath, such as the sheath 34 hereinafter described, may be sufficientto prevent vibration of the core laminations. Preferably, however, andas shown best in FIG. 4, the laminations are bonded to one another by abonding agent interposed between each pair of laminations.

In FIG. 4, the laminations of the core 26 are shown at 27, 27 and thelayers of bonding agent between each pair of adjacent laminations areindicated at 29, 29. Various different bonding agents and methods ofapplication and curing may be used. In

the presently preferred case, however, an epoxy resin, referred to asC-7 Epoxy Resin sold by Armstrong Products Co., IncQof Warsaw, Indianais used in combination with an activator sold by the same company andreferred to as H2O Activator." The resin and the activator are mixed byweight in the ratio of 29 parts activator per 100 parts of resin, or byvolume in the ratio of 1 part activatorto 3 parts resin, and the mixtureis then applied at room temperature to the laminations by painting itonto the laminations with a brush. The laminations are then stacked onone another to form the desired size core and are placed in a jig whichapplies a constant pressure between the top and bottom laminationstending to squeeze the laminations together. The jig and the laminationsare then placed in an oven or other suitable heating device and heatedto approximately 160 F. during a curing period of at least 1 hour. Theheating of the core during the curing process not only speeds up thecure, but also renders the bonding agent more viscous so that it tendsto flow out from between the laminations, to spread itself evenly and tofill up voids, leaving only a very thin layer of bonding agent betweeneach pair of laminations in the fully cured core. Before the laminationsare painted with the bonding agent, they are, of course, cleanedthoroughly to remove scale and dirt as by dipping in a suitable acidbath. After the finished core is cured, excess bonding agent is removedtherefrom before the insulation sheath, if any, is applied and the wireof the coil 28 wrapped therearound.

The wire from which the coil 28 is made is preferably magnet wire, thatis wire having a thin coating of insulating resin or the like, and ispreferably insulated from the core 26 by a sheath 34 of insulatingmaterial surrounding the core and located between the core and the coil28. The insulating sheath is not, however, necessary in all instancesand may in some cases be eliminated. On the other hand, in cases wherethe core 26 is made from laminations which are not bonded to oneanother, the sheath 34 may be used to aid in holding the laminationstightly to one another to reduce or eliminate vibrations of thelaminations. The sheath 34 may consist of a tape or web of insulatingmaterial tightly wrapped around core, but preferably it consists of aheat shrinkable tube of polyvinylchloride which is shrink fitted ontothe core so as to tightly grip the core and hold the laminations firmlyin place relative to one another.

The subassembly made up of the core 26 and coil 28, together with theinsulating sheath 34, if any, forms the basic electrical and magneticparts of the transmitting antenna, but it has been found that when anantenna consisting merely of such subassembly is energized by arelatively high power audio signal, as required to produce the desiredfield, the convolution of the coil and other associated parts externalof the core tend to vibrate relative to one another and to the core andproduce a hum, chatter or other noise which is often sufficiently loudas to be unacceptable. To eliminate this source of noise, the coil andcore subassembly is, according to the more detailed aspects of thisinvention, potted in a material which completely fills the otherwiseempty spaces between the individual coil convolutions and other parts ofthe subassembly so as to restrain the wire and other parts againstvibration and to dampen any vibrations which do occur.

As to the potting of the coil and core subassembly, the antenna 16 ofFIGS. 2 and 3 includes an outer shell of nonmagnetic material comprisedof a cylindrical plastic tube 36 closed at one end by a plastic end capor plug 38. The core and coil subassembly is located within the shelland the intervening space is filled with a potting material 40. Variousdifferent potting materials may be used but at present the preferredmaterial is a room temperature vulcanizing silicon rubber requiring acuring agent, such as sold'by the General Electric Company, SiliconeProducts Department, Waterford, New York, under the designation RTV-l 1.In FIGS. 2 and 3 part of the potting material has been broken away toshow more clearly the other parts of the antenna, but it should beunderstood that this material completely fills all of the otherwiseempty space within the shell. In addition to eliminating the noise whichwould otherwise be present, the potting of the core and coil subassemblyin a shell provides the finished antenna with a simple neat appearingcapsule shape which is easily handled, and it also protects the coil ofthe antenna against damage during shipment and placement.

In the process of potting the core and coil subassembly, an empty shellis placed in an upright position with its open end uppermost. A smallpiece 42 of cured silicon rubber or other similar material is thenplaced in the bottom of the shell, and the coil and core assembly isthen inserted and centered relative to the shell. The potting material,which is made by mixing a liquid base material with a liquid curingagent, is then poured in liquid form into the shell until it is filled.To assure that all of the spaces between the convolutions of the wireand other parts of the subassembly are filled with the potting agent,its curing is preferably performed under vacuum, the vacuum typicallybeing maintained for a minimum of about 6 hours. After the curing of thepotting agent the antenna is in its completed form, and in this form theinsert 42 located at the bottom or front end of the shell serves to holdthe adjacent end of the core 26 away from the end cap 38 so as toprevent chattering as a result of the core engaging and vibratingagainst the end cap.

FIGS. 5 and 6 show in general the manner in which the shape of the corecontrols the shape of the associated magnetic field. In these figuresthe shell and potting material of the antenna have been omitted forclarity. FIG. 5 shows an antenna having a relatively long core 26a incomparison to its crosssectional shape. As shown by the associated fluxlines 18a, the field produced by such an antenna tends to be relativelylong and thin with the field extending a relatively long distance fromeither end of the core. In the antenna of FIG. 6, the core 26b isrelatively short in comparison to its cross-sectional size and whenusing an antenna of this general shape the field, as shown by the fluxlines 18b, 18b tends to have a relatively by said antenna furthercomprising a quantity of nonmagnetic short and thick shape. Therefore,by-varying the length and/or cross-sectional size of the antennacore'its field may be varied so as to have a shape generally similar tothat of FIG. 5, generally similar to that of FIG. 6, or someintermediate shape. a 1

Also, by varying the power suppliedto the antenna the size of the fieldmay also be controlled. That is, as the powerto the antenna is increasedthe field projected therefrom tends to in crease in size, and as thepower is decreased the field tends to decrease in size, such increasesand decreases occurring along both the length and the width of the f eldso as to maintain the general field shape dictated by the core shape.The power which the antenna can properly handle is dependent upon itsimpedance which may be in turn varied by varying the number of turns inthe coil 28. In FIGS. 2, 3 and 4, the coil 28 has been shown to consistof a single layer of turns but where necessary to provide the properdesired impedance two, or more layers of turns may be used in the coil28.

Although the preferred embodiment of the invention has been describedabove, it should be-understood that various changes may be made from theconstruction disclosed, and that the drawing and description'are not tobe construed as defining or limiting the scope of the invention, theclaims which follow being relied upon for that purpose.

lclaim: d d

l. A means for establishing a magnetic field the frequency of whichvaries in direct correspondence with that of a given audio frequencyinput signal in an audio induction communication system whereinintelligence is transmitted solely by such magnetic field, said meanscomprising a source of an audio frequency input signal an audiofrequencypower amplifier for amplifying said audio frequency input signalfromsaid source, and an antenna connected with the output of said poweramplifier so that the signal supplied thereto is an amplified version ofsaid audio frequency input signal, said antenna comprising a coil ofwire wound on a core of magnetic material.

2. The combination defined inclaim 1 further characterized by said coreof magnetic material beingfmade up of a plurality of laminations ofsilicon grain oriented steel.

3. The combination defined in claim 2 further characterized by saidlaminations being bonded to oneanother by a layer of bonding agentbetween each pair of adjacent laminations.

4. The combination defined in claim l further characterized by said coreof magnetic material being a substantially straight bar of substantiallyuniform cross section throughout its len th.

5 The combination defined in claim 4 further characterized by saidantenna further including a sheath of electrical insulating materialsurrounding said core between said core and said coil.

6. The combination defined in claim 4 further characterized pottingmaterial surrounding said coil and core and filling the spaces otherwiseexisting between the convolutions of said coil.

7. A transmitting antenna for a communication system whereinintelligence is transmitted solely by a magnetic field varied at anaudio frequency corresponding directly with the frequency of an audibleinput signal, said antenna comprising a core of magnetic material and acoil of wire wound on said core, and a body of potting materialsurrounding said coil and filling the spaces otherwise existing betweenthe convolutions thereof.

8. A transmitting antenna as defined in claim 7 further characterized bysaid potting agentconsisting of a silicon rubber compound.

9. A transmitting antenna as defined in claim 7 further characterized bysaid core of magnetic material being a substantially straight bar ofsubstantially uniform cross section throughout its le n th.

. 10'. A transmit mg antenna as defined in claim 9 further characterizedby said core of magnetic material being made up of a plurality oflaminations of silicon grain oriented steel with each of saidlaminations extending along the full length of said core and having itsgrain oriented so as to be generally parallel to the longitudinal axisof said core.

11. A transmitting antenna as defiinediin claim 9 further characterizedby said core of magnetic material being made up of a plurality oflaminations of magnetic material bonded to one another by a layer ofbonding agent between each pair of adjacent laminations.

12. A transmitting antenna as defined in claim 9 further characterizedby said core of magnetic material being made up of a plurality oflaminations of magnetic material, and a tubular sheath of plasticelectrical insulating material tightly surrounding said core betweensaid core and said coil.

13. A transmitting antenna as defined in claim 7 further characterizedby said coil and core being received in a tubular nonmagnetic shell, andsaid potting material filling the space between said coil and core andsaid shell.

14. A transmitting antenna as defined in claim 13 further characterizedby said potting materialconsisting of silicon rubber.

15. A transmitting antenna as defined in claim 7 further characterizedby said core of magnetic material being in the nature of a substantiallystraight bar of a substantially uniform cross section throughout itslength and made up of a plurality of laminations of silicon grainoriented steel each of which laminations extends along the full lengthof said core and has its grain oriented so as to be generally parallelto the longitudinal axis of said core, said laminations being bonded toone another by a layer of bonding agent between each pair of adjacentlaminations, said coil and core being received in a tubular nonmagneticshell, and said potting material filling the entire space between saidcoil and core and said shell.

